Self-Employed? Not Hired.

Why do some employers have a problem with hiring self-employed folks? As long as their side business doesn't interfere with their full-time job, I say hire them. Gone are the days where you could retire from the same job you've worked at for 30+ years. If you ask me, some people are smart by having more than one income because what if they lose their job? If they have no other income coming in, they're out of gas. I think it's crazy that some employers expect you NOT to have no other income but theirs. It's like they want you to depend on them for your livelihood. Because most states have at-will employment clauses (you can be fired with(out) cause), it's almost like some employers want the control of being able to lay you off for any or no reason.